With reference to FIG. 5, conventional magneto-optical recording and reproducing devices (31) use a magneto-optical recording medium (21) composed of a laminated magnetic substance film (21b) having perpendicular magnetic anisotropy, and a protective film (21c) on the rear side of a substrate (21a). To record data to this magneto-optical recording medium (21), the magneto-optical recording and reproducing device (31) comprises the following components: a data generator (29), which outputs data to be recorded, a magnetic coil driver (28), which outputs a modulated control signal based on data from the data generator (29), a magnetic coil (27), which is driven by the control signal from the magnetic coil driver (28) and generates an alternating magnetic field, and a optical head (30), which emits a laser beam spot to the above mentioned magneto-optical recording medium (21).
The optical head (30) is equipped with a laser beam source (22) that emits a laser beam, a condensing lens (24), which projects the laser beam spot on the magnetic substance film (21b) of the magneto-optical recording medium (21) by condensing the laser beam, a beam splitter (23) which changes the advancing direction of reflected-light from the magneto-optical recording medium (21) and leads the light to a photodetector (25), and a photodetector (25), that detects the reflected-light strength and other data. The laser beam source (22) of the optical head (30) is steady DC driven by the laser driver (26).
When data is recorded to the magneto-optical recording medium (21) by such a magneto-optical recording and reproducing device (31), the laser beam emitted from the laser beam source (22) is condensed on the magnetic substance film (21b) of the magneto-optical recording medium (21) by way of the beam splitter (23), condensing lens (24), and the substrate (21a) of the magneto-optical recording medium (21). The laser beam spot is focused onto this magnetic substance film (21b).
Thereafter at the local area of the magnetic substance film (21b) of the magneto-optical recording medium (21) onto which the laser beam has been focused, the coercive force is reduced as the local area temperature rises, and the magnetizing direction becomes easy to change.
Therefore, if the alternating magnetic field, based on coded data output from the data generator (29) is generated by magnetic coil (27), the positive magnetized area (61-63), composed of a positively magnetized part, previously magnetized in a fixed direction at width d.sub.0, and the negative magnetized area (64-65), composed of a negatively magnetized part magnetized in the opposite direction to the positive magnetized area (61-63) onto the laser beam spot area, are formed on the magnetic substance film (21b) of the magneto-optical recording medium (21). This is shown in FIG. 6. Data recording and reproducing is done in accordance with the existence or absence of this negative magnetized part, that is, in accordance with the existence or absence, length, and location of the negative magnetized area (64-65).
However, the width of the positive magnetized area (61-63) and the negative magnetized area (64-65) formed on the magnetic substance film (21b) of the magneto-optical recording medium (21), changes. This change is due to changes of the laser beam output level emitted from the laser beam source (22) and changes in the magneto-optical recording medium's (21) sensitivity, or the like. Consequently, the width is not always the same.
For example as shown in FIG. 7, when recording new data, called "rewriting", to a data area recorded as having a width d.sub.0 for the positive magnetized area (61-63) and the negative magnetized area (64-65), the width d.sub.1 of the positive magnetized area (71-72) and the negative magnetized area (73-74), formed by new data, may become smaller than the width d.sub.0 of the previously written data in the positive magnetized area (61-63) and the negative magnetized area (64-65).
In this example, the residual positive magnetized area (61a-63a) and the residual negative magnetized area (64a-65a) exist on the magnetic substance film (21b) of the magneto-optical recording medium (21), mixed with the positive magnetized area (71-72) and the negative magnetized area (73-74) formed by recording new data. The residual area is larger in width than d.sub.1 of the positive magnetized area (71-72) and the negative magnetized area (73-74), located in the already written positive magnetized area (61-63) and the negative magnetized area (64-65).
The magneto-optical recording and reproducing device (31) reads and reproduces data in accordance with the existence or absence of the negative magnetized part, that is, or the existence or absence, length, and location, of the negative magnetized area. Considering the above example, when written data is reproduced by the magneto-optical recording and reproducing device (31), the residual negative magnetized area (64a-65a) which is a part of the negative magnetized area (64-65) formed based on old data, can possibly be reproduced as a negative magnetized area that exists a negative magnetized parts, together with the negative magnetized area (73-74) formed based on new data.
In particular, in a means where the signal data is recorded in accordance with the length of the negative magnetized area, a pulse width modulation (PWM) for example, since the negative magnetized area edge position is important, if such residual negative magnetized areas (64a-65a) exist, as above mentioned, this may result in the false signal generation or the incorrect detection.
Therefore conventional magneto-optical recording and reproducing devices have the problem reduced data recordation and reproduction reliability.